Punch riveting is a joining process performed with rivet elements. These rivet elements comprise full punch rivets and half-hollow punch rivets.
After the punch riveting, the punch rivet connection undergoes a quality check. One differentiates hereby between a non-destructive and a destructive quality check. Visual inspection, the check of the outer joint geometry and the process monitoring are commercially used as means for the non-destructive quality check. However, visual inspection only provides general conclusions about a produced punch rivet connection, since only outer characteristics of the punch rivet connection are available. In the case of a connection with half-hollow punch rivets, these include for example the concision of the rivet head, the state of the die-side sheet, damage to joining component surfaces by the hold-down device and the alignment of the rivet with respect to the die.
Even in the case of the check of the outer joining element geometry, only the variables of the produced joint connection visible from the outside are available. These are the rivet head end position, the bulge/upset dimension during punch riveting with half-hollow punch rivet and the embossing depth during punch riveting with full punch rivet.
Process monitoring based on the force/path data of the joining process is also used for the quality check. The force/path curve of a produced optimal joint connection is used as the reference curve for the evaluation of the joining processes. Envelopes, tolerance bands or process windows are placed around this reference curve in order to be able to determine a deviation of the force/path data from the reference curve during a joining process.
Another alternative for the quality check is the aforementioned destructive check of the produced joint connection. For the destructive quality check, macro grindings of the joint connection are prepared and/or strength tests of the joint connection are performed. An evenness of the joint parts in the joint zone, a seam formation between the joint parts, a concision of the rivet head with a punch-side sheet, an undercut formation and a lack of cracks in the joint connection can be evaluated from a macro grinding. The mentioned strength test enables conclusions about the bearing capacity of the punch rivet connection under shear, peel and head-pull stresses.
In practice, the joint parameters and the geometric variables for the joint connection are normally determined in preliminary tests. On this basis, the rivet head end position and the bulge/upset dimension of an optimal joint connection are taken as the reference variables, since they can be determined in a non-destructive manner. The effort of the destructive quality check is thereby reduced. But these reference variables must also be measured individually after each joining process. This is associated with a lot of time and is not suitable for series production. Another alternative is the random-like check of the above reference variables.
Thus, the object of the present invention is to provide a method for checking the quality characteristics of the joint connections, which is improved compared to the state of the art.